Water quality: hydrogen sulphide strip test: practice question

A PSM question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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PSM · Water quality: hydrogen sulphide strip test

Sample item

After floods, a village has a jaundice outbreak. The orthotolidine test shows no residual chlorine at the tap. With no laboratory available, the medical officer wants a simple field test for faecal contamination of the water sources. Which test is appropriate, and what does a positive result look like?

AHorrocks test; the water turns blueBThree-tube multiple-tube method in the field; gas in the tubesCOrthotolidine test; the water turns yellowDHydrogen sulphide paper-strip test; the bottle turns black

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

D. Hydrogen sulphide paper-strip test; the bottle turns black

The hydrogen sulphide strip test is a low-cost field test in which faecal bacteria that produce hydrogen sulphide blacken the medium within about 24 to 48 hours at ambient temperature. It is a presumptive test for faecal contamination. It can be used when laboratories and incubators are unavailable.

Option by option

A. Horrocks test; the water turns blue
The Horrocks apparatus estimates the bleaching powder needed to disinfect water, not contamination.
B. Three-tube multiple-tube method in the field; gas in the tubes
The multiple-tube method needs a laboratory with incubation and reagents.
C. Orthotolidine test; the water turns yellow
The orthotolidine test measures chlorine and has already shown that no residual is present.
D. Hydrogen sulphide paper-strip test; the bottle turns black (correct)
Correct. A black precipitate in the bottle indicates presumptive faecal contamination.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

Questions about this PSM question

What is the answer to this PSM question?

The best answer is D, Hydrogen sulphide paper-strip test; the bottle turns black. The hydrogen sulphide strip test is a low-cost field test in which faecal bacteria that produce hydrogen sulphide blacken the medium within about 24 to 48 hours at ambient temperature.

Why is "Horrocks test; the water turns blue" not the answer?

The Horrocks apparatus estimates the bleaching powder needed to disinfect water, not contamination.

Why is "Three-tube multiple-tube method in the field; gas in the tubes" not the answer?

The multiple-tube method needs a laboratory with incubation and reagents.

Why is "Orthotolidine test; the water turns yellow" not the answer?

The orthotolidine test measures chlorine and has already shown that no residual is present.

What concept does this question test?

It tests Water quality: hydrogen sulphide strip test in PSM. It is a presumptive test for faecal contamination. It can be used when laboratories and incubators are unavailable.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Water quality: hydrogen sulphide strip test?

Practise biostatistics with small calculations until sensitivity, predictive values and risk measures are automatic. Then use the flashcards and the other PSM questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.